Atrial Flutter
A macro-reentrant circuit where the wavefront chases its own tail around a massive, fixed anatomical obstacle. The right atrium becomes a racetrack.
The right atrium is supposed to be a quiet reservoir. It is a gentle collecting pool for deoxygenated blood returning from the body, contracting once per beat to top off the right ventricle.
But under the right conditions, it transforms into a racetrack.
If a premature beat arrives at the exact wrong millisecond, it can trigger a macro-reentrant circuit. Unlike AVNRT, which spins in a tiny microscopic loop hidden inside the AV node, this circuit is massive. The wavefront chases its own tail around a huge, fixed anatomical obstacle. It travels in a continuous, unyielding circle, overriding the sinus node entirely.
This is typical atrial flutter.
The Anatomy of the Racetrack
To understand the circuit, we have to look at the boundaries of the right atrium.
Anteriorly, there is a giant hole: the tricuspid valve. This is an electrically inert ring of fibrous tissue. The electrical impulse cannot cross the hole. It must go around it.
Posteriorly, there is another barrier. The crista terminalis is a thick ridge of muscle that runs down the back of the atrium, flanked by the vena cava.
Between these natural boundaries lies a broad track of continuous atrial tissue. The wavefront travels down the lateral free wall of the right atrium, hits the bottom, and squeezes through a narrow corridor. This critical choke point is the Cavotricuspid Isthmus (CTI). It is a strip of muscle bordered by the inferior vena cava on one side and the tricuspid valve on the other.
Once through the CTI, the wave sweeps up the interatrial septum, over the roof of the atrium, and back down the free wall. Round and round it goes.
The Sawtooth Wave
On an ECG, atrial flutter is famous for its "sawtooth" pattern. But why does it look like teeth on a saw, instead of distinct P waves?
In a normal sinus rhythm, the atrium fires and then rests. During that rest, the ECG baseline returns to a flat, isoelectric line.
In atrial flutter, the atrium never rests. The electrical vector is constantly changing as it spins around the huge tricuspid annulus. Because the wave is always moving, the electrical vector is always moving. There is no flat baseline.
In classic counterclockwise flutter, the wavefront travels up the septum. The inferior leads (II, III, aVF) look up from the bottom of the heart, so they see this massive wave moving entirely away from them. This creates deep, negative, plunging sawtooth waves in the inferior leads, and positive waves in lead V1.
The AV Node's Role
This racetrack is incredibly fast. The wavefront typically completes a lap in 200 milliseconds, meaning the atrium is contracting at an astonishing 300 beats per minute.
If the ventricles tried to contract at 300 bpm, the heart would not have time to fill with blood. Blood pressure would collapse.
Fortunately, the AV node is standing guard. As we learned in Volume II, the AV node exhibits decremental conduction. It acts as a physical filter, protecting the ventricles from dangerously fast atrial rates.
The AV node simply cannot recover fast enough to let every flutter wave through. It naturally blocks every other beat. The result is a 2:1 conduction block: an atrial rate of 300 bpm, paired with a steady, controlled ventricular rate of exactly 150 bpm.
In the EP lab, we map this circuit by pacing the heart. We pace directly from the Cavotricuspid Isthmus (CTI) at a rate slightly faster than the flutter. This is called "entrainment."
When we stop pacing, we measure the time it takes for the flutter to return to our pacing catheter—the Post-Pacing Interval (PPI). If the PPI exactly equals the Tachycardia Cycle Length (TCL), we have proven that our catheter is physically inside the circuit.
The cure is remarkably elegant. Because the entire macro-reentrant circuit must squeeze through the narrow CTI, we simply burn a line of scar tissue across it. This is like building a wall across a racetrack. The wavefront hits the wall, the circuit breaks, and the flutter terminates instantly.
Key Takeaways
- Mechanism: A macro-reentrant circuit traversing the right atrium around the tricuspid valve.
- Anatomy: The wavefront travels down the lateral wall, through the Cavotricuspid Isthmus (CTI), and up the septum.
- ECG: Continuous undulating "sawtooth" waves without an isoelectric baseline, classically negative in II, III, and aVF.
- AV Node Filter: The atrium fires at ~300 bpm, but the AV node usually limits conduction to 2:1, resulting in a ventricular rate of 150 bpm.
- Cure: Ablation of the CTI creates a line of block, permanently interrupting the circuit.